I think it is the ServletRequestclass object which is used to store attributes in request scope. I refer to the following context from SCWCD Kit Book, Section 12.3, pg218.
Data is shared between servlets using the three container objects, ServletContext, HttpSession and ServletRequest. The JSP technology, since it is based on servlet technology, also uses the three scopes, which are referred to in JSP pages as application, session and request scopes.
Any verification on this matter is welcome. Thambi